Can one give a gift to a friend for buying a house with a riba-based loan?

If the matter is as described, and he has committed this grave sin and insisted on it, despite your advice to him, then you should not accept his invitation to celebrate his buying this house, or give him anything on this occasion, when in fact he needs to be offered condolences, not gifts, because riba is a major sin. Allaah says (interpretation of the meaning): 

“Allaah will destroy Riba and will give increase for Sadaqaat (deeds of charity, alms). And Allaah likes not the disbelievers, sinners”

[al-Baqarah] 

“O you who believe! Fear Allaah and give up what remains (due to you) from Ribaa (from now onward) if you are (really) believers.

279. And if you do not do it, then take a notice of war from Allaah and His Messenger but if you repent, you shall have your capital sums. Deal not unjustly (by asking more than your capital sums), and you shall not be dealt with unjustly (by receiving less than your capital sums)”

[al-Baqarah 2:278-279]

And Muslim (1598) narrated that Jaabir (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) cursed the one who consumes riba and the one who pays it, the one who writes it down and the two who witness it, and he said: they are all the same. 

And the Prophet (peace and blessings of Allaah be upon him) said: “A dirham of riba that a man consumes knowingly is worse before Allaah than thirty-six acts of zina.” Narrated by Ahmad and al-Tabaraani; classed as saheeh by al-Albaani in Saheeh al-Jaami’, no. 3375. 

You should carry on advising him to repent to Allaah from what he has done. 

We ask Allaah to forgive us and him, and to set the affairs of all the Muslims straight. 

And Allaah knows best.