I've been thinking about the opposite of this a lot, as I do every summer -- about how I feel it's *not* considered socially acceptable to complain about feeling uncomfortable or even ill in the heat in public, so I never do.
You may feel nauseous, or like you are about to pass out and can't breathe well. You may be embarrassed because your neck/chest is red and splotchy from heat-rash, or because you're the only one in the room whose face is damp, but the only "right" thing to do is smile and agree when people talk about how beautiful the weather is.
I think that, at least in most areas of the US, hot weather is something you are "supposed" to look forward to and agreeing about how nice it is is supposed to be a human bonding experience. So, not being physically well-suited for it makes you an outcast. People can be very judgmental about it.
I've never noticed this with cold weather, though. I don't think most people look down on those who can't handle cold temperatures well as being unfit or lazy for not wanting to participate in certain activities or be outdoors.
I think that complaining about being snowed in or how a heavy rain affects people's plans, etc., is a way for people to connect through small talk, and is looked upon as normal.
I'm tempted to just suggest they make the most of it since it won't last long and before we know it, they'll be lamenting the end of the heatwave and wishing it were hot again! !
But really what can I say?!
I think a cheerful, "Oh, it won't be long until Fall's here again!" would be fine. I wouldn't add anything that assumes they will just wish it was hot again, even if you really are talking to someone you know well enough to believe that is true.