Given what our mc in this is doing to get off, I'm genuinely surprised. Clearly it had occurred she needs to be extremely careful not to let her roommate see her 'strap-on' for obvious reasons. Given how she has even verbally expressed a willingness to do something sneaky like that and actually do it, the fact she went with that option knowing all it would take was one mistake or even a completely unforeseeable variable for it to end in a complete disaster is again very surprising. Surely she would have at least thought of other options to avoid that. For one she could have taken a path where she ended up learning things about her roommate from routine to some potential dirty secrets (maybe the roommate would turn out to be highly attracted to futa girls), and then take advantage of that by 'accidentally' letting roommate see her showering or changing in her room or some other stuff. Do subtle stuff to get her to make the first move instead, this time knowing what she's getting into. In that event the worst possible thing that could happen is that mc would have to wear a condom or take responsibility.
Of course there's no guarantee roommate would be attracted to that. Hell she might even be potentially disgusted by futa which would give even more need for mc to not let her see the 'strap-on' between her legs. But since there's still a massive risk of following the original path this game does. Not really fool proof approach to being sneaky, but there is an option with a lot more insurance for her to try... Roommate lives under the same roof, so our sly futa mc could also try just waiting until the middle of the night, creep into the bedroom and carefully undress her. Then she can proceed to blow however many loads into and/or onto roommate until satisfied, afterwords she can do what she needs to to cover her tracks and get out of there before she gets caught.