I used to keep mine in a community tank but now frequent forums which recommend against it. Surprised that the frog is being bullied tbh, they are really friendly, is the current owner sure it's aggression? Mine occasionally grab onto each other out of excitement at feeding time but that's because their eyesight is poor and they don't always recognise the other frog as a friend not food.
The main issues people tend to encounter with keeping these guys in community tanks are not getting enough food (this can be overcome with target feeding) and fertilisers/medications being toxic to them, which would cause an issue if you ever needed to medicate the guppies for any reason.
They do also like to be in groups so something to consider in future if you do take one in.