Was this discrimination?

[HurdyGurdy19]

This is not my scenario, but that of a friend. The friend has MS and has returned to their home country for three weeks for medical tests and treatment.

They applied for a job with a local authority but the interview panel were not able to offer them an online interview. The interviews were only being held on a particular date, which fell when the applicant was abroad for these tests/treatment. The interview panel said that to offer an online interview would be unfair to the other candidates.

Given that the applicant was abroad for tests/treatment directly relating to their disability, could this be seen as discriminatory?

No.

They have opted to be out the country, whether for botox, an MRI or a pedicure, and weren't available for the interview. That's the end of that.

They should make adjustments for disability, but not necessarily for being abroad. Had she been at home would she have been able to attend?

Potentially. The Equality Act applies during recruitment processes and MS is a recognised disability.
The date of the interview arguably put your friend at a disadvantage compared to other applicants because she was receiving treatment/attending appointments for her disability on that day. If her decision to be abroad at the time was purely her choice and not necessary, this may be more questionable.
If she had to be abroad, the question then is whether it would have been a reasonable adjustment to interview her on a different day, or remotely.
There may be perfectly good reasons why allowing it remotely or on a different day wouldn't be reasonable, but the employer should have considered it.

Thank you for the swift replies.

The interviewer did say that if the other candidates are unsuccessful they will go back to them, so there may still be a small chance.

Did they state the interview timetable in advance, so that she knew when she applied she would not be available?