First step is to do a soil test. May need lime or be low on needed fertility requirements. fertility levels, or lack thereof, can lead to the unintended creation of favorable growing conditions for the weeds over that of the grass. Just because it is a pasture does not negate the necessity of fertility management. Don't look over noting what kind of weeds are actually growing and the conditions they prefer, this can lead back to fertility/Lime program. Don't forget to check some of the micro nutrients as well.
Type up three proposals. When submitting them to the property owner list them in order of cost and approximate time frame for results.
Proposal #! apply Gly and reseed with acceptable grasses.
Proposal #2 fall plow, winter fallow, spring tillage and seeding of acceptable grasses.
Proposal #3 Monthly mowing, or sooner depending on the type of broadleaf growing, to keep weeds from going to seed.
Add in fuel and labor costs for each proposal. I don't want to force someone into something they don't want to do but provide all the information so they can make a decision based on time frame and facts. If more money goes into your pocket because someone doesn't like herbicides, so be it. They are happy and so are you, which is the ultimate goal. Everyone is happy and relations remain intact to continue doing business.
While the chemical method is the fastest, cheapest, retains the most deep soil OM, and will not effect any part of a healthy soil cycle be prepared to point out the drawbacks of the other methods. Plan #2 will be time consuming, require tillage which will lead to soil erosion, quickly burn up stored OM which will effect water retention during dry times ultimately effecting plant growth when needed most. Also one should point out the soils natural glues binding particles together will be disturbed which leads to erosion. Erosion is not just from water run off but by wind as well.
The third option retains benefits of plan 1 but is likely going to take the longest as the weeds already present have certainly went to seed previously and will take some effort until grass gets thick enough to out-compete the weeds.
Good luck