I believe the game picks whichever card is the center of your friend's colo team.
If you're not satisfied with your back up. You can simply press back and reselect the opponent to reroll your friend's help card. It's a strategy to keep rerolling until you get a friend card that can counter your team's counter.
Example, you run a full BABE team which is strong against MOE but weak against SEXY. You can keep backing out on the opponent until you get a MOE back up card. So if one of your BABE cards lose against a SEXY it can immediately be countered by your friend's MOE card for a high chance to win in that lane (Your opponent can still get a back up in that lane to counter your assist card).