Tribunal hearing is critical to attend by the home purchaser in order to pursue your right.
What if the purchaser is not available during the hearing date?
The purchaser can assigned some one who is the family members or spouse to attend on behalf.
I doubt that friend can be attend on behalf as I personally witness friend is not allowed to be represent purchaser during the hearing day.

What should I do to appoint my spouse or family member to attend on behalf?
If you cannot attend and you will need to write a letter and get the Commissioner for Oath for stamping.

Below is one of my closed friend who is not able to attend the Tribunal hearing schedule on 15 Nov. He went to the Pesuruhjaya Sumpah (Commissioner for Oath) at Kulim to write a letter.
Both parties need to be present to show the NRIC. He shared that the clerk will assist to draft the letter with their format. The cost of the letter is RM16.
Below is the sample letter for reference