Hard one. I'm on my phone so will be brief, but a skinny cat with a bad coat can also be a giveaway sign of a thyroid issue which is very common in middle to older age cats. Guess I'm just saying it might be best not to jump to conclusions (mind you I haven't seen the cat).

If it was me I would just make a passing comment to the owners 'gee she's a skinny little thing isn't she' and see how they respond and take it from there...