Either they have problems or looking for a fresh blood.

If you going to advise = fix a monthly fee. Advise is an opinion don't tie it up to percentage of income. Maybe your advice will not have an immediate effect. Ask 100% payment in advance/ or 50% before 50% after

If you going to be hands on = Grow their business and take a bigger chuck of growth income. Say that they are making 100K/month. Grow it to 300K and take 35% of it.. play with your ratios