The airlines sell points, often with a bonus offer. The price isn't great but possibly worthwhile if using for first class tickets. But there is a annual limit, typically 50000 or so. If you are trying to accumulate United points, you can buy them for 1.9 cents each using the mile accelerator program, this is a fairly good price.

The person going with you can try to accumulate points, it might be easier to try to get 100000 points in each of two accounts rather than 200000 in one account. Potentially your partner could go the credit card route and get the miles. Or if you buy the miles, you may be able to get 75000 or so for each of two accounts in that way.

Don't assume that first class tickets will be available just because you have enough points to buy them; recently it seems to me that the points can be very difficult to use. The airline that you accumulate points for may not have any first class flights on offer for the day, week or month that you hope to travel. If you have not yet chosen an airline or alliance for your trip, consider availability and how flexible your trip plans are.