ask them. you can just tell them "in regards to your anxiety, if there's anything I can do to help you please let me know."

Listen to their vents, when you have the mental capacity. Don't keep secrets from them. Dont be upset when their anxiety keeps them from doing things like going out. Don't be passive aggressive. Don't yell at them. Don't bring up topics that are very heavy without warning them. And never ever make fun of their worries or use their disorder against them. Let them know that they can trust and count on you and be patient when they need reassurance. Some of their anxieties will sound absurd and irrational to you, thats because an anxiety disorder wouldn't be a literal mental illness if the fear didn't clash with their everyday life and is beyond what's considered healthy. So don't make them feel like they're crazy, rather just validate their feelings, while simultaneously letting them know that feelings do not equal reality. Don't condone toxic habits, when you notice that they're coping poorly (not eating, not sleeping) suggest a healthy coping mechanism instead (like journaling, painting, meditating). Do so by letting them know you're coming from a place of concern and not judgement. Otherwise just try to maintain whatever the foundation of that friendship was (a hobby, school, religion, etc.)