Just did a bit more math here.
Given the numbers above, if the company is valued at $100m and not adding shares, the valuation could be ~$5.60 per share.
When buying ISO's you need to pay for the shares and pay the taxes on the difference between the strike price ($0.27) and the fair market value on the date of purchase. Therefore, in this example, tax would be owed on ($5.60 - $0.27) * 15000 or $79k in paper gain. Depending on other income and a variety of factors, tax due may be greater than $28k.
Total cash out of pocket to exercise these options would then be the amount paid for the options plus the tax.
$4050 + $28k for a total of $32k on a valuation of 15000 * $5.65 or $84750.
If Gitlab does not decrease in value, he could benefit about $50k on an investment of $32k. If Gitlab increases in value, the benefit could be larger.
Not a tax guy here so corrections are gladly accepted and appreciated to increase the clarity of the situation.