His friend promised to pay off his debt after one year; will this affect his Hajj?

Who has guided you to adhere to Islam. What you mention will not affect the validity of your Hajj. If you meet the conditions that mean you are able to go, then go for Hajj. “Being able to go for Hajj means being physically healthy, having the means of transportation to reach the House of Allah by plane, car, riding-beast, etc., according to one’s circumstances and having sufficient provision for the journey there and back. The money that you have to cover your Hajj expenses should be surplus to whatever is required to cover one’s financial obligations [spending on family members and others who are under one’s care, etc.] until one gets back from Hajj.” Fataawaa al-Lajnah al-Daa’imah, 11/30.

You have to repent to Allah for helping your friend to do something haraam. Allah says (interpretation of the meaning):

“… but do not help one another in sin and transgression …” [al-Maa’idah 5:3]

And you have to explain to your friend the ruling on this matter, and advise him to repent and not repeat that sin. May Allah give us and you the strength to do that which He loves and which pleases Him. May Allah bless our Prophet Muhammad.