Don’t do it. sdr is right (can’t believe I just typed that!) – that is what credit cards are for.
I am currently involved in starting a new business with my existing business partners. The person who is putting it all together and will run the new company is someone I have known for almost 20 years. This will be our 3rd company with the same partners. The 1st is the company that I currently run and the 2nd we sold last year. I completely trust this person. I guarantee that there will be signed agreements in place and attorneys involved before I am asked for one dime. It is absolutely necessary to protect all parties.
I would not enter into a partnership with your friend without seeing his company’s financials. Don’t sign anything or give him any money until you know exactly what you are getting into.