I'm 29. I have two non-millionaire friends who are 27-28 years old and have over 100k in savings each. They saved the money over last 5-6 years by being smart with their finances. Both are willing to invest $20k-$30k in me whenever I am ready. Neither expects crazy high returns (10% interest rate will suffice). They also know that if I hit it big, I will reciprocate well beyond 10%. Both trust me enough that no matter what happens in my life, they will at least get their seed money back in a couple of years.
Even though 25% of savings seems like a big bet, if you truly trust a person, it really isn't a risk. I wouldn't trade my friends for any amount of money so cheating them out of a measly $20k during a stage in my life when I really need their support would be pretty stupid of me. I have life insurance and so if I get hit by a truck, they will be compensated well. I don't see why it isn't bright to invest in your friend's startup if you trust them. Sure, money can get in the way of friendship but as long as there is honesty and maturity among all parties involved, investing can only make the bonds stronger.
My friend loaned me about $10k over the course of a year and I fully paid him back once I was able to. This helped strengthen our friendship instead of tearing it apart. So my personal advice is to go ahead and invest in your friend if you trust him/her. You only have one life and if 25% of your savings can change their lives forever, you gotta give it a shot.