I'm going to step down from my friend's wedding 2 days prior
Why do you need to wait two days to step down from your friend's wedding?
I just found out that my dad doesn't want me to go to the reception.
It is understandable if you have a reason not to go to a wedding, but it will be rude for you to tell you can't go 2 days prior. They have set everything and planned for everything so you should tell you're not coming as soon as possible so that they would know what to do.
It's the friend's thing. If they are close, they will get over this decision.
Even so, they may feel very blindsided by your decision and may ruin your friendship over this, and you may regret it in the end. I understand that it is not within your control that you can't come, but it is well within your control to tell them many days in advanced. I hope you reconsider and tell them immediately you can't go.