1. Involving your friend in your wife's employment is a huge no-no. It only makes you more dependent on him, and he certainly knows this. If he places your wife in a job, don't be surprised to see him leveraging this in the worst way possible if things don't work out as expected.
2. You are maximizing a stake that isn't yet worth anything, and may never be worth anything. 20% of $0 is $0, just as 25% of $0 is $0. Perhaps a better way to look at this: figure out how much this venture would need to be making before the total value of your compensation package (equity and any eventual salary) would exceed what you're already making now at your six-figure job. The number is probably much larger than you think it is.
3. If your compensation is stock granted outright for services, you should be aware that this may create undesirable tax consequences for you. You should speak with an experienced accountant about this.
4. The fact that your friend needs to continue working to fund the venture is a huge red flag. What if he loses his job? What if he has a medical emergency? What if his wife divorces him? If you're going to get in bed with an "investor", he should be putting in most if not all of the cash up front. Which begs the question: if your friend says he's prepared to invest $15,000/month for 10 months, is he capable of transferring $150,000 to the new company tomorrow? It doesn't sound like it.
Again, the risk you're concerned about is a result of the fact that you are pursuing a deal structure that makes no sense. I would highly recommend retaining competent legal counsel before you go any further.