there's a fiddle and accordian group that meets in a local pub, and I see an announcement for the Pipe Band and Fiddle Club, 2 Bells road, Stornoway meeting on Monday nights, 8.30-10 pm. If you make contact via the e-mail addresses given at that website, you can probably get inside info for what's on during the time of your friends' visit.

sometimes An Lanntair art gallery, Stornaway Town Hall hosts concerts
on weekends there may be ceilidh dancing and singing at the British Legion or the Yacht Club; usually listed in the Stornaway Gazette

also, if they can head out to Nis(Ness), check out Taigh Dhonnchaidh