Conceived Times After Having Sex?

If I slept with a girl on the 13th May (I used a condom) and she's now 6 weeks pregnant but slept with someone else a few days/week later (But she says he was high & the condom fell off during) who is more likely to be the father? She doesn't remember when her last period is or when she conceived.
Update: She also does not know when she was ovulating either
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