You don't mention whether or not your friend has a mortgage on Property A. If so, the lender would object because, in lender-lingo, a three-way split of Property A's well/water rights would "impair" the lender's collateral.

From a property-value perspective, your friend's property would decrease in value once a community-well arrangement was consummated. Conversely, B & C would increase in value thanks to the agreement.

Were I your friend, I wouldn't entertain B & C's proposal under any circumstances. Owners B & C bring nothing to the table, yet have everything to gain. OTOH, your friend gains nothing, yet gives up something of considerable value.

B & C have demonstrated that they have neither the money nor the willingness to maintain their already-existing wells. Think we can predict how they'd handle the shared responsibility of maintaining your friend's well and paying for their monthly water usage.

The money it would cost B & C to: 1) pay your friend a lump sum up front to compensate for lost property-value; 2) pay all attorney fees for having a lawyer (who specializes in such matters) research applicable laws and draw up a proper agreement; and, 3) pay the cost of materials/labor ... would probably cover the expense of bringing their existing wells up to speed.

In summary, bad idea!