You have 34 acres between a potential of 100 large adult sheep over winter which is about 3 and a half sheep which is ok. However you will need to lay up some land for hay/haylage from January to get a good cut and know how the grass crops and how much you will need to not have to buy in...which is v expensive at the moment.
With lambs and rotating fields and resting, therefore cutting down on the imputs from wormers, hard feed etc which all cost money you have just about enough for the sheep. However the cattle will eat lots of the grass and make it unviable to run healthy sheep with out high inputs which will ultimately reduce your profit.
It depends on a lot of things...
How intensive do you intend to be i.e are you going to push the land to its limits, How much time do you have, how much feed do you want to buy in, Do you want to finish your lambs on grass quickly or have to feed them, Do you have buildings....there is a lot to consider however and others will have their opinions. At the end of the day though profits, sheep, humans and importantly the land will have to cope and a low input higher output system is actually more profitable and sane that a potential high input high output system that has a lot more scope to go pear shaped!
Tell him the cows have to go...but he can run a few heifers for a few weeks if sheep numbers are down. Sheep and cattle complement each other but dairy cattle require more in way of nutrition than sheep.... if you had fewer sheep and allowed 1 beef cow to the acre for the summer you could do it but you need to be in control of both species to keep the balance right...