I just purchased a new S&W M&P40. It did not come with a manual thumb safety, and my wife is very adamant about one if she is to ever conceal carry. I am willing to give the plastic caps as well.

Has she ever carried before? Is she trained in handgun use and stress situations? Very easy to forget to cycle the safety off in a self-defense combat situation.

Keep the M&P for yourself and get her something smaller and lighter with a thumb safety, if she's absolutely insisting on one. Most women have smaller hands (Bruce Jenner notwithstanding) and the FS M&P will be uncomfortable/unwieldy for them.