In a situation like this, with a woman I do not know well and who has not shown up at a meeting asking for advice and breastfeeding support, I would not offer any advice at all unless the mother has specifically expressed that she is upset about having to supplement and wishes she could just nurse 100%. And then I would only mention that an IBCLC or LLL leader may be able to help, if she has not already received help. If she responds with enthusiasm to that, I would offer to email her contact information if she wants me to. (okay, I might mention oatmeal cookies in a joking way, but only if this lady seemed really receptive and might like an excuse to eat cookies.)
If the mother is not openly saying or displaying that she is upset about supplementing and wants to change it, I don't have any reason to assume that she would want to change it. Many mothers are not comfortable with 100% breastfeeding and are actually relieved when someone tells them they "have to supplement". While I am not happy with the misinformation that leads to this situation, I don't feel like it's appropriate to go around giving unsolicited parenting advice to strangers who are probably perfectly happy with the way they are caring for their child. I would not hesitate to offer support and information to someone who asked for it or expressed a desire for it, and I would not hesitate to enlighten someone who was offering "advice" to me concerning how I feed or care for my own child...because if they have done that, the ball is in my court and a response is appropriate. But I would not initiate a discussion about it involving someone else's child unless there was a really clear green light to offer support, and even then I would be very cautious.
The other situation I am thinking of is a mother who really does not want to be supplementing, and would love to breastfeed 100%, but has already been through everything an IBCLC has suggested to try and it has not worked, or she has reached her own personal limit for how much effort is too much (perhaps nursing with supplementing device, and pumping multiple times each day, and supplementing formula, constantly weighing baby, etc) and has finally decided to nurse as much as she can, supplement the rest, and relax and enjoy her baby, knowing she is doing her best.....but is still sad about it...for a mother like this, a comment from a pro-bf stranger who does not know her whole story, and may accidentally say something that might come across as judgmental or condescending....can be really upsetting.
The last reason I would not offer any advice or comments unless it was really clearly desired is because of the impression it can make about the stereotypical group of "breastfeeders". I have overheard quite a few conversations between mothers who talk about how turned off they are when they are on the receiving end of a pro-bf lecture from someone they don't know and didn't ask for advice from. I think the negative feelings about the "lecture" and the "breastfeeder" (or other term) who gave it outweigh the benefits of any information they might have received, and they are left with the idea that they don't want to be "one of those (insert term)". I think giving out unsolicited advice about infant feeding probably does more harm than good.
In most situations I think the best response is to smile and tell her that her child is beautiful. If you make a friend who wants to get more support for breastfeeding, that will come up again.