What is the ruling on dealing with someone who has taken out an interest-based loan, and is it permissible to visit him and attend the events he hosts?

Your friend must repent to Allah for what he has done, by feeling remorse and resolving not to return to it. He must also repay the money borrowed from the bank to avoid usurious interest. As for your relationship with him, it is governed by the benefits and harms that result from it. If maintaining the relationship achieves a benefit, then do so. If shunning him leads to his reform, then shun him. Eating at his place and accepting his gift varies in حكم (ruling) depending on his state.