You can invite anyone you want. But there is no such thing as an "invitation letter" or any other type of invitation that would grant her, or anyone else access to coming to the U.S. If she wants, she can apply for a visitor visa, but she must apply and be approved based on her own merits, and her merits alone. The chances of her (or anyone) getting a visitor visa from Lagos are just about zero, add in that her spouse lives here in the U.S, that brings her chances to zero. Just stating the facts, the only thing you have to lose is the fee. If it were me, I wouldn't waste my time, or money.