Can we assume that BWI-IAH is on the same ticket and same PNR as IAH-SYD?

It's a tough call and what UA will do could be limited if none of her flights is cancelled yet.

IMO it's a risk that IAH might be getting wide body and longhaul flights in and out roughly on time while smaller domestic planes are delayed. Two hours there isn't much time if there are severe weather problems. OTOH a 6 am departure would be miserable even with lounge access. If she can get to IAD relatively easily, I'd try have the tickets changed to depart from IAD nonstop to IAH on a relatively large airplane with some additional hours cushion at IAH. Otherwise, look for something similar from DCA or even PHL/EWR (since those airports are easy to reach by train). Remember too that space is very dynamic in these situations and flights into IAH that are full might have seats available if one asks again a few hours later.

I assume that if the IAH-SYD flight is cancelled, there's no way to get to Australia without losing a day (too late to fly to SFO and catch that flight) and moreover space will be limited. You might want to try finding some alternative *A routings through Asia or NZ to be prepared with Plan B, C, D......