There is never the right time, or the right words, to tell someone that they're overweight. No one, in the history of the world, who was ever told "you're fat", responded by saying "gee thanks, I didn't know that I'll lose weight right away!". They already know they're fat; being overweight doesn't equal being stupid. They know that their clothes don't fit them too well any more and they feel short of breath when they go upstairs. They don't need it pointing out to them; it's their body; they know it better than anyone and they'll already be having self esteem issues. Please don't add to that. The only thing you can do is, if you usually take a treat for them when you meet up, don't take cakes, biscuits or chocolate. Other than, leave them be.