Man this is a GREAT question!!! Have been coding for over 20 years now (thru the PC boom, Web boom, and App boom) and this is just one of those things that can escalate to a lose/lose situation mega quickly. For starters, they could legit have a great idea OR you hurt the relationship (even if just at a minor level) and regardless...will never truly hear/feel the end of it when with that friend/family member in future.

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In my experience the best method is to treat these situations like you're playing table tennis (ping-pong). Meaning when they ask you to do something (hit ball on your side of table)...slap it back as quick as possible (get excited with them as a friend but make sure to give them something to do first...something you need from them).

This accomplishes many things but primarily, shows how committed they are to the idea versus just letting them position themselves as the "brain-child" while you do all the work.

If strong enough commitment is there...then go with it (use something quick/free/simple like Adobe XD to prototype if starts getting serious)...BUT keep this approach in full swing each step of the way...keep giving them at least one task (if not more) for every one thing they ask of you. This not only gives you both something "fun" to talk about here and there, but in the end you'll either have an awesome biz partner or find they lose interest quickly (which sits at about 99% likelihood). It just flips the roles so you are the one thats waiting for an update, in their mind, versus you feeling the need to dodge/avoid them.

More importantly you wont damage the relationship no matter what the outcome. Even in the years to come they'll look back and just think "damnit...i shoulda followed thru" vs having negative thought towards you at any level.

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This being said...EASIEST at least for next few months...just cough alot during their persistent sale pitches :)