Asking a loved one to aid in childbearing is the ultimate compliment and the ultimate favor. As the initiator, examine the big picture as well as the here and now. The challenges of infertility can cloud judgment with desperation, and bad communication leads to hurt feelings. It's the grown-up version of the Golden Rule and it's true of every relationship, whether that's personal or professional, romantic or platonic. Recruiting a friend or family member in conception is no different.
Presumably, your intended surrogate is someone whose character and values match up with your own. We're also assuming they meet the physical requirements for surrogacy. Ideally, this is a loved one with whom you share a bond that has faced and overcome difficulty before with open, assertive communication. A loved one who's been open in the past about very personal issues, such as their sex life and own health (maybe even too open), is also more likely to be transparent about lifestyle choices during pregnancy. A friend close enough to be blunt with you and know the relationship will survive may feel more intimidating to ask, but an honest answer is worth it.