PVI is a brilliant philosophical theologian, but his comparison with Mother Teresa is inapt:
1. The question at issue is not whether Muslims can refer to the same God as Christians. Some Muslims have studied Christian theology, and when they attack Christian theism they are referring to the Christian God.
The real question is whether Muslim theism is coreferential with Christian theism. To begin with, Muhammad apparently had no firsthand knowledge of the Bible. So even if he thought he was talking about the same God, that doesn't mean he knew what he was talking about. That doesn't mean he successfully refers to the same God. In addition, Islam has developed in conscious opposition to Christianity.
To take a comparison, does Starbuck in the original BSG refer to the the same character as Starbuck in the reimagined BSG? The new Starbuck is clearly inspired by the old Starbuck. In some respects it's a similar role with a similar function. However, one is a male character while the other is a female character. That, in itself, is a major discontinuity. In addition, the psychology of the new Starbuck is so different from the original character that they aren't recognizable the same person.
My point is not to give a definitive answer but to illustrate the difficulty with making claims about identity when there are such pronounced differences. Even on the most charitable interpretation, the relationship is far more ambiguous than PVI makes it out to be, with his facile comparison.
2. Furthermore, there's a distinction between referring to the same God and worshipping the same God. A Muslim apologist can refer to the Christian God but that doesn't make him a worshipper of the Christian God.
