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One of two things is going on here. 1. This fella is overly concerned about what other people think about her weight and how it reflects on him, or 2. He is genuinely and personally turned off by her weight gain. If his problem is the former, he should snap out of it and get some mental healthcare for that. If it's the latter, everyone has different likes and dislikes, and they're entitled to them whether anyone else agrees with them or not. Clearly this is becoming a problem for him for whatever reason, so he needs to figure out WHY this is a problem and take appropriate action, whether it's seeing a therapist or cutting her loose so she can find someone who will want and love her even if she's overweight.