My understanding is that you can’t share an account with a friend as it’s not allowed by the EULA. If the cost is a big issue, you are allowed to keep using whatever you downloaded even if you cancel your subscription. In other words, you and your friend could both subscribe, download what you need as individual subscribers and then cancel your subscriptions before the month is up.
Quoting the FAQ: “When you cancel your subscription, you won’t receive access to future releases of Unreal Engine 4, however your login will remain active, and you are free to continue using the versions of Unreal Engine 4 which you obtained as a subscriber under the terms of the EULA.”
There are some exceptions for universities and such, but my recommendation would be to read the EULA properly, and then decide how you want to go about things. You could for example choose to subscribe a few times a year to get updates if you’re not working on anything too serious, or even choose to only subscribe once if you mainly want to see what it’s all about. If you’re actively working on a project though, I would highly recommend both of you spending the $19 a month as it’s a great deal. You have over a hundred people working on Unreal Engine 4 all the time, so the updates and such is well worth the asking price IMHO.