EC had alcohol addiction during the 70's and 80's.
He's sober since November 87.
I believe, Mark invited him to fill Jack´s place and help EC
at the time EC had already invited Mark to join him in 87 and early 88, so I don't think that Mark "helped" EC...
If I remember well some MK interviews, BIA aftermath was not an easy period for Mark.
At that time Mark was a very big star that you normally don't see in the role of being someone else backing guitarist (except for "one off" such as Prince's Trust). So I think that being this deluxe guest was for him a sort of way to breathe.