We've all heard people say that politics and religion are two subjects we should never discuss with our friends. It's time to put another topic on that list: WEIGHT!
So much of what we say and feel is relative to our life experience-- in essence, our truth. What we believe is too much (of anything) may not be enough for someone else. The best we can do is be a good listener for our friends, and in essence try and be a sounding board rather than believe it's up to us to validate or disagree with their view of themselves. There are ways to be supportive without getting into "who's right".
At the same time, if your friends bring up topics/issues that for whatever reason are uncomfortable for you, you have the right to let them know and hopefully they will respect your wishes and not bring the subject up again. Relationships are about give and take, being considerate and wanting the best for everyone involved.
By the way, you mentioned you really like your daughter's mother-in-law. I'm happy for you and I know what that feels like. My son was recently married, and his in-laws are wonderful too! I feel fortunate for him and us!