My neighbors bought a big house five years ago on credit and they are paying interest until now. Can they go to the Hajj knowing that they bought their house on credit?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Debt is of two kinds: either due now (at the present time), or delayed. If it is due at the present time, then your neighbors have to pay it. After paying their debts, if they possess money by which they can afford to perform Hajj, they should do so. Allah Says (what means): {And [due] to Allah from the people is a pilgrimage to the House –for whoever is able to find thereto a way.} [Quran 3:97]

In this case, they may ask permission from their creditor, if he permits them, they can go to Hajj, otherwise no.

If the debt is to be paid in installments and they have money to pay those installments, or what is due of them, and their Hajj will not lead to delaying the coming installments, then they may go to perform Hajj.

Finally, we would like to draw our brothers' attention that your neighbors should make a sincere repentance from dealing with Riba (usury/interest). This should be done before going to Hajj. Riba is one of the grave major sins and one of the seven  destructive sins and Allah has declared war against those who deal with Riba.

Allah Says (what means): {O you who have believed, fear Allah and give up what remains [due to you] of interest, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal - [thus] you do no wrong, nor are you wronged.} [Quran 2:278-279]

Allah knows best.