Just the presence of a supporter does help a lot! To me, small things a support could do are so much appreciated, valuable.
Grrl77 you could help by knowing the techniques used by your partner/this site as there are plenty and procedures well documented and quite extensive. Just to be sure no important information gets lost. Busting for example has a very specific procedure where timing is essential. Sometimes it is hard for a cluster head to follow the instructions point after point (slapback headache, ch becoming even more severe but less frequent may force somebody to resist...)
To me nothing is more sad than sitting on a chair in the shower using hot/cold water fighting the beast, knowing nobody but really nobody is there, just me alone...
Just the presence is important. I know and my ex wife knows she can't help on the pain so I never ever did but weight on here shoulder. But a warm hug, cup of tea, whatever after an attack is so much appreciated and can't be paid with money.
But please, as supporter do not make the mistake going into suffering yourself! I know this is difficult but important, very important supporters controlling the impact to themselves as hard as possible.