If he asks the current GF to marry him, will she thereafter humor him and his desire to have more knives? Or will she think that he can live without a Mk3, because she needs a new ___ (whatever).

However, if he finds a new GF and hindes from her his collection of knives and penchant for evermore, then maybe he can convince her to buy him a Mk3.

I'd take my chances on the one woman who already bought him a knife. The next woman could be someone who asks why he needs a weapon like that and who he wants to go stab.