For a 2-4 unit property you will need a minimum of 25% down for (most) conventional financing (20% for SFR's) this will have to be sourced and seasoned for 60 days, your max CLTV (Combined Loan To Value) will be 75%.
What does this mean?
Source and Seasoning the Down Payment: Conventional lenders (for the most part) will require that you verify, that, for AT LEAST 60 days, you have had: either the funds in your account OR you have earned (maybe a bonus at work, business profits, ect) or are entitled to funds (retirement savings account, inheritance, award prizes, proceeds from the sale of an assett, ect) and can prove where you obtained those funds from to show they are not a new debt or ineligible funds (funds that can't be sources or borrower funds).
As to the LTV/CLTV: you will not be able to have the agreement between you and your fathers friend recorded against the property at time of closing, because of the CLTV restrictions, you can borrower 60% from conventional financing and get a 15% 2nd mortgage from your fathers friend, but, this would not normally make sense to do since the 1st mortgage is willing to go to 75% anyway. The restriction requires that you as the buyer has "skin in the game" to ensure that the 1st lien holder is not the only one at risk. In theory you could record the agreement after the fact, but, you do increase your risk of the lender calling foul and possibly being accused of fraud (not saying you would be convicted or charged, but it is possible), depending on how soon after the closing you record the agreement and when the agreement was dated, ect....
Always good to have the agreement in writing and witnessed/notarized is a good idea, recorded against the property is much safer for the lending party, but may make the financing of your intended property a bit more difficult if they expect to have this done through the closing of the loan and you are using a conventional lender. I can't tell you how to structure the deal so you can defraud the lender you will be using (I don't wish to put any licensing at risk), but, I'm sure there is a way to accomplish your goals if you were to structure this correctly, now, that you know what the problems are, that you are going to face.