if the loner’s Conflict Goal is the same as that of the team, I believe that technically the only option is to have the loner be part of the team, and the loner’s player can just chose to not accept help from others on the team, as well as not offer any helping dice to the rest of the team on his Actions. The “loner” part of it would be primarily roleplay. For game mechanics purposes, the loner would share in the team’s Disposition, as well as the results of the outcome (compromise, conditions, etc.).
If the loner choses a different Goal, you could possibly break him off into his own team. According to the rules, however, when you break into multiple teams, I believe you’re supposed to make them each of them approximately the same size.
Note: This “I’m too much of a loner to be part of a team” situation can make for interesting roleplay… but probably not for very long. I hope that the player of the loner character is interested in eventually growing out of that trait (or at least such a strict interpretation of the trait), and that it makes for an interesting story arc for his character, rather than an ongoing nuisance for the patrol as well as the other players. I guess I should also point out that if the loner is a tenderpaw, he’s likely to not get promoted to full guardmouse by his companions/elders, which means likely expulsion from the Guard by the time you reach the Winter phase. If he’s already a ranked guardmouse, I hope that you as GM spent some real time with his player discussing how and why he could be so detached from his team, and what it’s going to take to get him to work, at least somewhat effectively, with his group. I also hope that the other players at the table are genuinely ok with the situation. Otherwise, no one’s going to have a good time.
If this were my game, I probably would not let this go as far as the Winter phase. If the other players did not sort out the loner (probably via some “learning life lessons” and “tough love” during the Players’ Turns – aka. PvP), I would try to point out to the loner that teamwork is pretty much essential in Mouse Guard. I would not do this necessarily via a conversation so much as having the loner mouse get separated from the group, and put into some serious trouble. Possibly even killed. The world of Mouse Guard is dark and dangerous, and mice (even with swords) are still amongst the smallest (and tastiest) of creatures in the territories.
If it’s a problem with the player, not the character, then that’s a different situation. Any advice any of us could give you if that’s the case is just conjecture, and a stab in the dark, but I guess I would offer up this: Mouse Guard is not necessarily a game for every type of RPG’er, and if the player of your loner refuses to conform (at least somewhat) to some of the game’s core conceits, there are probably better games that your group could play. Or, you could ditch the player.