Hello @The7onWalnut thanks for your post. This is a great question because as a small business owner myself, it's always a struggle to try and build up an email list. Here is just one article that discusses potential legal issues regarding building/using a mailing list for your company. If you consider you're "buying" your friend's list, you may be "legal" but I have a better solution for you.
Why not capitalize on any good will that your friend's customers have for their business, and if they are willing, I would ask if they would send an "intro" email to the list telling them about your store. There may be a greater chance that email would opened/read instead of the recipients getting an email from a strange company. The intro email could contain some selling points, why your store is worth a visit, and maybe a special offer or coupon. In either the email or when they redeem the coupon, ask them to join YOUR email list to continue to receive special offers.
I hope this helps answer your question, and maybe gives you a few new ideas to try.
