A small amount of pre-ejaculate or semen could be sufficient, provided it came in contact with the inner labia when it was moistened, but the main question is what point of her cycle she was at at the time of the encounter. Other factors could be penetration (how far in) and location. Still, have her take a morning-after pill (provided her consent on the moral standpoints) or other contraceptive means. I would consider her chances of pregnancy equal to that of a single act of unprotected sex, potentially 33% That number may be high in this case. (FAQ:
http://www.avert.org/faqpreg.htm#q4 from AVERT.org)
For future note, it may be wise to purchase a personal lubricant containing Spermacide (Nonoxynol-9 probably the safest) and incorporate that, as well as washing prior to the encounter. Some people place a condom over their finger prior to fingering to prevent bacteria from being transmitted that may lead to UTIs or other infections, and if that's the case, be certain any lubricant you purchase is water based, not oil based. (Oil based lubricants may compromise latex barriers, i.e. condoms) It may break the mood for a moment, but it's a bit safer.
Hope this helps.
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35yrs • M • 
