Absent a written agreement, the parties to an easement share the cost of maintenance and repair.
In this case, the party with the "dominant parcel" (the one receiving the water) is going to be responsible for the cost of the water pipes (they cannot require the servient parcel owner to pay for their water supply).
The cost of repairing the driveway following damage to the driveway secondary to the pipeline repair however is going to be split between the owners. The Dominant Parcel has an obligation to make reasonable repairs, but this cost of repair does not increase with improvements made by the Servient Parcel. (So while the servient parcel may have a right to have the property returned to the condition it was originally, they are not entitled to additional paving, or other surfacing, at an increased cost.
However, as a practical matter, if this cost is relatively low (most contractors will figure this into their service), the servient property will cover this cost as well - it is only when the dominant parcel owner becomes overly greedy (asking for excessive repairs (or even improvements to their land) that most disputes arise.
If you cannot reach a resolution between the two property owners, you can sue one another in small claims court and get what is called a "declaratory judgment" where a judge will issue a ruling telling you exactly what each party owes under the terms of the easement.
Short of filing a lawsuit, you can try to mediate the dispute with them - contact your local bar association and request referrals to mediators, a third party neutral can often help you reach a mutually agreeable resolution. Use the bar association's referrals to contact a mediator or two, the mediator will then contact the other party to set up a mediation session, and you can go from there - hopefully resulting in a formal or written settlement agreement, and save yourself the time and expense of litigation.