The kefir grains are a culture of bacteria and yeast that are active at near-room temperatures. Their ideal temperature is 71F (22C). Below this and they will grow too slowly. Above this temperature, up to 86F, for extensive periods the grains will be damaged.
Building the grains takes time- there's no way around it.
You can keep the grains at exactly 71F to maximize their growth. You can also change the milk often and keep the milk to grain ratio high to ensure that the milk doesn't acidify too quickly and the grains stay active.
You may see an improvement using organic milk as it won't have residual antibiotics in it.
I have not read anywhere of a way to modify the milk itself to encourage growth. You might experiment with adding a small amount of milk powder to make the milk more nutritionally dense but my gut feeling is that this would have a negligible effect if any.
I'm afraid your friends are just going to have to wait patiently.