Dried fenugreek is quite bitter. You need to see what TYPE of Fenugreek helps milk production. Herbs have different properties depending on how they are processed. Fresh fenugreek will be different from dry fenugreek. Boiled fenugreek will be different from soaked in the secondary fenugreek. Or, it might be chew on the fenugreek.
I did a quick test, it is fenugreek seeds: Breastfeeding Online
It can be taken as a powder, capsules, or tea. As a tea form it is less effective. However, if you want it in the beer, I would consider putting it either in the secondary or the last 2 minutes of the boil before chilling.
As for using the fenugreek, taste it first. See how bitter it tastes. If you are curious what happens when you boil it make some tea and taste that, see if you want that flavor added to your beer, experiment. I would consider lowering the amount of hops in exchange for the bittering of the fenugreek.
Alternatively, you can add it to the secondary. You might want to look at older recipes that used herbs instead of hops for bittering and preservation. I would use up to 10oz in the secondary, but maybe less depending on how fast the flavor leaches.
Good luck, let us know what you do and how well it works out.