Gaining

Unfortunately, you have to take your chances. Mobility depends on weight, age, and any conditions someone may have. You probably won't have problems at 400. But not everyone can stop gaining. You may remain strong, or you may get bad joints. No way of telling.

As you point out, one can have mobility problems at any weight, but the risks increase as you get bigger.

Personally I wouldn't want anyone I cared about to get too big to get around easily. It's easy to cross the line, become immobile and not be able to lose the weight. Your bf's ex is not the only one this has happened to.