I am planning an HBAC and happen to also be studying to be a doula, and this is what I would say, have them BOTH. I think the friend will be great support for the two of you, especially your hubby, but when planning a VBAC, I would want someone there who has worked with VBACs before. There are a lot of emotional hurdles that your friend has probably not gone through, and may or may not deal with them as well as an experienced VBAC Doula. I think it really depends on your hospital too. If it has a VBAC rate of less than 80%, all the more reason to hire a VBAC Doula, as she will better ensure success in a maybe not so friendly VBAC environment.
There is a lot more to a Doula than having given birth before, and I don't know your friend personally, so I couldn't say how much she knows about specific situations you may encounter (baby funny positioned, prolonged labor, fast labor, etc) Personally, if any of my friends were pregnant right now, even with me having done all my studies, i would still recommend then hire someone with more experience for the professional support. Of course I would be there for them as well, but there is so much more to a having Doula support, that I feel a friend can't bring to the table. The same reason, I feel most dads can't be Doulas.
Again, I don't know your friend, but I'd hire someone in addition to her.