Obviously, If you do this the way you've described, the party will figure out what's going on in about five seconds. To prevent this, there's a number of different issues you need to tackle, so let's go through those before discussing solutions. (I'll assign your actors psudonyms for ease of reference: Your force user with the mentor is "Urist McPadawan," the other party members are "Johnny and the Castaways," and the absent, projecting mentor is "ForceMaster32." We good? Good.)
So, like I was saying, if you tell Johnny and the Castaways that Urist McPadawan is talking to himself, they'll start investigating immediately, because that's what players do when they spot an obvious plothook. They'll probably start by asking Urist who he's talking to, and Urist will waste no time asking ForceMaster32 why this is happening.
If you don't mention that Urist is talking to himself, your players will later want to know why; Players expect to be told about things that are blindingly obvious, unusual, and potentially relevant their survival. Also, the minute your players start planning or getting tactical, they'll start either incorporating or ignoring ForceMaster32 in their plans, and will discover the discrepancy in their perceptions anyway. On that note, if you use roll20's combat grid, the presence/absence of the mentor from the combat will tip your players off pretty quick. It won't be long before Johnny pops the obvious question, and the whole thing falls apart.
You'll need to come at this from a number of different angles. Here's some possibilities for you to mix and match:
- Make Urist McPadawan's conversation with ForceMaster32 look less obvious than someone talking to himself on the subway. One easy way to do this would be to make Urist's half of the conversation also take place inside his head, but be prepared for your players wanting that force power eventually; Silent telepathic communication is one of those things players get a lot of use out of.
- Come up with a reason why Johnny and the Castaways aren't able to see Urist McPadawan talking to himself. One option is to split the party, but this has well-documented downsides. Another would be to give Urist the Expanded Universe equivalent of a Bluetooth headset, but then you'll need to convince Urist to wear it without explaining why, which might be tricky.
- Prevent or discourage Johnny and the Castaways from investigating why Urist is talking to himself. This is almost impossible if Johnny and the Castaways and Urist McPadawan can freely communicate, but constantly interrupting them or splitting the party could both potentially work.
- Prevent Urist McPadawan from noticing ForceMaster32 isn't contributing much. If ForceMaster32 is in the habit of being useless already, this will be easy, but if she's normally throwing force powers around with aplomb, you'll need an excuse for why she isn't doing it today. (Alternatively, if you let her do things while remote projecting, you'll need an excuse for why she doesn't just work from home all the time.)
- Prevent Johnny and the Castaways and Urist McPadawan from noticing the other keeps weirdly assuming that ForceMaster32 is/isn't present during planning
and discussions. This is going to be hard, but splitting the party might work.
- Prevent your player characters from simply asking ForceMaster32 what's going on when they're not all on the same page. This could allow you to maintain some of the mystery even after the discrepancy is discovered. If Forcemaster32 is in the habit of lying or being annoyingly cryptic, this might be easy; Otherwise, you'll need to manufacture some reason why ForceMaster32 can't tell the party right now... Such as being suddenly very busy trying not to die.
My recommended solution, though, is this one:
- Let one or both sides into the ruse, either in or out of character. Honestly, your plan will be a lot easier if the player in control of Urist McPadawan knows his character is operating under a false assumption, or if Urist McPadawan knows that he should duck away from the party for a moment if he want to talk to ForceUser32 without seeming crazy. Alaternatively, you could just let both sides know there's a weird discrepancy in their perceptions of realiy, and let them have fun playing that out. (One of my favorite sessions ever involved an invisible demon telepathically talking to one party member at a time; The players had great fun discussing what was going on both in- and out-of-character.) I don't know if your table goes in for those styles of play, though.