Ankylosing spondylitis

My dad has been diagnosed with Ankylosing spondylitis. He occasionally goes through periods of severe lower back pain lasting a week or so but feels no discomfort following that for a months. He is 47 years old and has never done any resistance training.
Is it advisable for him to begin resistance training? If yes then are there any modifications to be made or things to consider for exercise selection, programming, etc.?
​​​​​​This is my first post so please ask me for more details if the information I have provided is inadequate.
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Yes, we would advise that he begin resistance training.

Regarding the question of modifications, the same principles discussed in this post apply: Resistance Training CP - Training Q/A with Drs. Feigenbaum & Baraki - Barbell Medicine Forum

Thank you!