Ethereal Jaunt can't take another creature
Technically you've already departed from RAW by allowing the PC to travel to the Ethereal Plane with the phase spider. The Ethereal Jaunt ability does not grant the ability to transport other creature along with the spider.
That said, it's a fun house rule, and now that you've made it, I would roll with the situation and make this a side quest to find a way to return this character to the Material Plane.
Getting back by magic
There are two spells that could potentially rescue someone from the Ethereal Plane. Both are 7th-level spells: Etherealness and Plane Shift. Unfortunately Etherealness despite the apt name is unhelpful here as it states:
This spell has no effect if you cast it while you are on the Ethereal Plane or a plane that doesn’t border it, such as one of the Outer Planes.
There are a couple of magic items (Oil of Etherealness and Plate Armor of Etherealness) that are also unhelpful as they mimic the spell and therefore have the same restriction.
That leaves Plane Shift as the only real viable solution. Fortunately, it can be cast on a group of creatures so either another member of the party or a helpful NPC can help if the PC doesn't have access to it (which is likely if they are fighting phase spiders).
Getting back as a quest
Instead of focusing on a fairly mundane mechanical solution, turn this interaction into an adventure. Come up with some interesting tasks or challenges the party needs to overcome to rescue this PC. Either have the party find a way to get to the Ethereal Plane or have the stranded PC explore the plane for a way to escape. Either way, have the players whose characters aren't present play as some temporary characters or NPCs so that they aren't left out while this is resolved.