Divide the number of shares your friend is getting by the total outstanding. Then multiply that ratio by the valuation, and you've calculated the "value" of the grant. With the following serious caveats:
* Almost any pre-IPO company is going to eventually issue more shares. (Post-IPO, too.) This will dilute existing stock and option grants. Typically companies do refresher grants when this happens—to employees who are still present and performing. Unless it's in your employment contract, you are not entitled to automatic compensation for your existing shares when this happens. The board could issue another 50 billion shares tomorrow and give them all to other people.
* Private valuations are often fairy tales; it's very common to see these slashed in half or more overnight, especially in a business environment where a company needs to keep raising cash. Companies that are undervalued at the $70B level are few and far between—there's a reason we call them "unicorns."
Your friend should 100% talk to a lawyer (not an accountant) about potential taxes. It'll probably cost about $2-4,000 and could save them 100x that much.