In general, borrowing money to invest is a risky proposition for a very simple reason: investments can go down in value, while the loans you had to take to make the investment absolutely do not go down in price if the investment didn't work out. So you have a guaranteed cost, but never a guaranteed return.
That said, there are a number of red flags here beyond the general facts of investment.
For one, a "family friend" is always a warning sign, because its an overly optimistic way of saying "someone I don't personally know very well". If there are people in the family who know them personally enough to be a "friend", and not a "family friend", then they are in a far better position to judge if they should lend money to them - so let them be on the hook for $7k if they are so trustworthy.
Second, an accountant is a professional position, and should be making a large enough salary on their own that investing or getting loans on the order of a few thousand dollars should not be a stretch. If they need to ask you for $7,000 to invest, this strongly suggests they are not in a position to be trusted to handle your money.
Third, "guaranteed return" is a huge red flag, because in this situation there is 0% meaning to this statement. Even if they had access to an Employee Stock Purchase Plan where they could purchase stock at a discount (and the guarantee is not on the return on investment here either, and any guaranteed is only to them - there is no possible guarantee to you, other than their promise to pay you).
If their promise to pay was of so much value they could get a loan from a bank, or cash advance on their credit cards, for this amount of money as an unsecured loan and then they would get to keep 100% of the profits. That they have exhausted easier sources of funding suggests anything they promise you is likely to be based purely on "gee, I hope they pay me back" and that's about it.
Given this, it is extremely disturbing that a financial professional such as an accountant would claim that lending them money in such an unsecured fashion could be called having minimal risk or a guaranteed return, and this suggests they are either incompetent or fraudulent. Sure, there is probably little risk to them, but that's clearly not how you were supposed to interpret any such statement. In neither case would it be a good idea to give the person money.
Finally, never risk money you cannot afford to lose. Given the fact that you do not have $7,000 to invest, you cannot afford to lose such a sum by definition because you do not have it - and borrowing it is not the same as having it. If the deal goes bad, how long will it take you to pay off that loan - how unpleasant would that be, and how long would you need to be reminded of their dishonesty and your mistake?
I can think of an awful lot of better things to do with $7,000, and I bet you can too!