There are several ways to induce lactation and entire websites devoted to it
But when you say little girl, how old? It's only worth trying with a very young baby (newborn to a couple of months old at the most). All the information you'll get on adoptive breastfeeding is American, where babies are adopted at less than a week old usually. Once they are past that, you aren't really as likely to succeed - first off, the baby needs more milk, and inducing lactation is not as likely to give a normal supply. There are many women who have to supplement with bottles even newborns. It can be stressful, and believe me, adopting is hugely stressful anyway.
Also, if your friend is in the UK, I really don't think it can happen anyway. Babies tend to be older (5+ months?) and you need to be keeping their routine as close as possible to what it was before. It will be a very stressful time for the baby when everything around them is changing. Moving a child is traumatic, even when they don't always show it. It helps to keep things the same for them. Same brand of formula for instance, and the same washing powder for clothes etc etc. An older child might not be happy about changing what they're eating
And lastly, social services (I can say almost without doubt), would not approve at all. The baby is legally like a foster child when it moves in, so she won't legally be the mum. I highly doubt she would be allowed to breast feed at all, and by the time adoption finalisation happens the baby will be too old. They just wouldn't let her do it, and they have to visit her several times to check how things are going.
Sorry I can't be of more help. If your friend is adopting an American baby/is American or similar, you can have a look at the websites out there - putting 'adoptive breastfeeding' into search engines will get you the best information