The way you phrase this question suggests that you see the powers of a witch as something separate from him/her as a person but in point of fact this is not the case. Think of witchcraft as more like a talent or a skill that develops with time and practise. You wouldn’t “steal” a person’s ability to read or to sing and in the same way you cannot steal the experience and knowledge a witch has developed over the years.
That said, you can weaken a witch. Witches like anybody are susceptible to issues in their private lives - illness, depression etc. A manipulative person wanting to reduce a witch’s casting power might look for ways to distract their attention. Often a focus on mundane life can affect a witch’s ability to perform. I know if I have financial problems or work problems my ability to concentrate and therefore to cast is usually affected. But this is not the same as somebody “stealing” power it is just a natural reaction in any witch who is trying to balance her mundane life with her magical one.
If I’m really honest I think this chap is giving you a bit of a line because he fancies you. Full marks for effort on the romance front but a bit more research about witchcraft needed!