She had also Severus with her since it was Sunday and the man visited his godson, in order to spend some time with him before he returned back. He would be able to provide proof about Petunia, Vernon and in extension to about Margaret Dursley. The Potters would be able to use the excuse that they didn’t know, what the Dursleys had done, but if Severus talked about Petunia, then things would get very nasty for them, because Lily wouldn’t be able to disprove this since Severus was her childhood friend and neighbour.