Seriously ladies...the request is about how to wean NOT a free-for-all for you to judge this mother on her decision to stop breastfeeding and make her feel badly about her decision (which you probably know nothing about).
S. - I'm by no means an expert on the subject, but have heard from my pediatrician that it is okay to mix formula and breastmilk. Some mothers will start with mostly breastmilk and a little formula, then cut to half-and-half, then mostly formula with a little breastmilk. Some mothers will give BM and formula in separate bottles as not to lose or waste any of the breastmilk. From what I understand, both methods are fine.
You know your baby, your body, and your life the best - I'm sure you will make a decision that is the most appropriate for your situation. By the way - your baby will turn out just fine even if he does end up on formula, so don't let others guilt you about that.