Well some downloading sites I use give you a custom download link that I assume must be IP address specific as sharing that link will not work, so my stupid question is:
Is it possible to 'spoof' you IP address to a custom one (ex. friends) so that when the download site generates the link it will be linked to that IP, then the person who actually has that IP can download it? Or is it just not worth the work it would take?

No.

Learn how IP actually works; google and wikipedia are your friends.

It would depend upon the model of the modem you and your friend are using and maybe in what country they are being used and by what service provider .it would be possible for you to sniff out his MAC address, then make some further modifications to your modem attaching an additional chip which would allow you to change the config file, which would allow you to set his MAC address to your modem, ending up basically making a clone.

I would go on, as there is a lot more to explain.. but somehow I have the feeling you no longer want to do what you were originally thinking of.