Did my friend's failure to fulfill her vow – fasting three days at the beginning of pregnancy and three at the end, and sacrificing two sheep – cause her to lose the fetus, and is she still required to fulfill it after losing the fetus?

A conditional vow is disliked, but it must be fulfilled if the condition is met. If the vow-maker is unable to fulfill it, an expiation for breaking an oath (kaffarat yamin) becomes obligatory upon them. If your friend's inability to fast her vow was due to a valid excuse, then she must offer expiation for breaking an oath and there is no sin upon her, and there is no connection between that and her miscarriage. However, if she had no excuse, then she has sinned, and there may be a connection between the miscarriage and the non-fulfillment, and she must seek forgiveness, repent, and expiate her vow with an expiation for breaking an oath. The apparent meaning of the vow is that it was conditional upon the pregnancy itself. Thus, it must be fulfilled, unless her intention was for the pregnancy to be completed with childbirth. In that case, the intention specifies the vow, and nothing is owed by her after the miscarriage.