I'm seeing an OB and a homebirth midwife at the same time. I do feel that I get better prenatal care from the midwife, although not knowing your client's miscarriage history (and what the reasons for the miscarriages may be), it's hard to say whether an OB would be helpful or not. There are women with clotting disorders, progesterone issues, etc. that sometimes require medication just to stay pregnant. So if that were the case for this woman, seeing an OB wouldn't be a bad idea. OBs *can* do more than just an u/s to see if baby is alive, once there have been multiple miscarriages and a cause is known (usually through blood tests done on the mother before getting pregnant again).
Anyway, as for whether to tell the OB or not... I told my OB. For one thing, I don't believe in lying. For another, I feel that if my OB can't accept my birthing decisions, I don't want him/her to be my care provider. My midwife agreed with me about telling him, as she said that SHE likes to know when someone is also seeing an OB - it goes both ways, ya know? And as it turns out, while talking to my OB, I found that at least 2 or 3 women had "accidentally had their babies at home", when they were really planning it all along and didn't tell him (he had heard through the grapevine before hand that at least one of the women was planning this). His feelings were hurt by that - that they weren't honest with him about their plans. But then again, my OB is not your average OB - he's actually very open minded and tries to best accomodate a woman's wishes, as long as he thinks it's medically safe to do so (and if he's not sure of safety, he does some research).
If I make it to 36 weeks (my previous birth was preterm), my midwife can deliver me, and I can stop going to the OB and have my wonderful out-of-hospital birth. If I go early, I have a good, open minded, NCB-friendly OB that I won't have to argue with to get the birth experience I want. I've done the randomly assigned OB thing in the hospital, and don't recommend it! But I also recommend seeing the midwife through the whole pregnancy, and just using the OB in tandem for as long as needed.