I personally love travelling in this area during rainy season. Most often it's just more overcast during the day vs bright sunshine, and a bit showery especially in the afternoon. Sometimes heavy but short downpours. Of the places mentioned, In Burma only Yangon and coastal areas are really affected. Yangon isn't a problem, stay out of beaches and Sittwe area during July, and concentrate upcountry. Upcountry Burma (Bagan, Mandalay, Inle, etc.) gets relatively minor affect from the monsoon, at least not enough to have a meaningful effect on sightseeing. Also not so crowded and prices usually down.

Angkor is lovely in the rainy season, green and jungly and not so dusty as in winter high season or March-May horrible hot season. Again, not so many tourists and prices down.

Vietnam South gets monsoon affect but probably nothing the average tourist can't handle. This is mostly Saigon end of the country, Mekong Delta, Phu Quoc Island (I'd avoid the latter in monsoon time). Vietnam Central and North is more affected between October-December, not July. Different directional monsoon.

So while she may ultimately decide on Bali, panning the original places on the basis of weather was bad advice.