My first time was in the next room listening to him bang her for a good few hours over and over. It was extremely exciting but very difficult at the same time. The sex parts are pretty great but for me the hardest bits were in between when they were lying together talking etc.
The first time is probably easier if your not there at all but I don't think it would be as exciting, at least in my opinion.
Unfortunately watching is something I've yet to do as its hard to find a willing person so I'm not sure on that.
My humble opinion is it would be worse. first time or not, to have to wait downstairs.
Her fucking an ex-bf who can make her cum is a good thing; watching would be better. Like onenut mentioned, the next best thing would be in an adjoining room so you can hear the noises, moans, and dirty talk. If neither of those are possible, negotiate being able to wait outside their bedroom door.
Either way, prepare yourself for frequent, intense masturbation.