The three Warhammer 40k RPGs (Dark Heresy, Rogue Trader, Deathwatch) share a somewhat common ruleset, and I believe this question applies to all three.
Can I move through an ally in combat?
The following response was from Mack Martin of Fantasy Flight Games:
This is a tricky question to answer, as the game isn't distinctly designed with battle mat play in mind. While a grid can certainly be useful in a game (and add some fun strategizing into combat) it would really depend on how much space a single square represents. In my games (and generally speaking) if a square was 3 or more meters then I would declare that you can move through allied squares without issue. If it were less (such as 1) then I would suggest that the character must go around. I would also say that if forced too, a character can squeeze through a space occupied by a friend (if each square is 1 meter) by spending 3 meters of move distance to get through.
3 metres per square feels like a lot of space, a 1 meter square, however, might get out of hand if a character can run 18 meters in a round. This is really something the GM and Players will have to figure out on their own.
None of this is official, obviously, but hopefully these guidelines can help your group come up with some suitable house rules for using a grid mat during your game sessions.
Not a firm answer, but it will have to do.
I'd just treat it as a dodge roll to get by, at the better character's dodge, and count it as 2 squares instead of 1.
Looking in Deathwatch, I can find nothing to indicate otherwise. However, if one wanted to be thoroughly pedantic, one could declare engagement on the friend, and then disengage, making it a half-move or move followed by a disengage (which includes a half-move).