it may be best to text under the circumstances as phoning my develop into a row
there is nothing wrong with texting, any more than writing him a letter would be wrong
but, dont use text speak, write it in a polite manner, not too blunt as it can seem rude in a text, but asking him outright what you want
something simple like
hiya, just wondered if you were still coming to the gig? id love it if you were but i understand if you would rather not at the moment, but let me know as its a shame to waste the ticket and jane likes singer too and would love to come. thanks bambi :o)
if not could you just be straihgt with your mate, tell her the truth, so she knows that she may be let down at the last minute... what plans must she make? are they ones that once made will be a pain to cancel?