I have mapped out larger plots of land we were looking to buy and definitely can get very close if you use Google Earth polygon measuring tool like @Frank Homes already suggested BUT you need to remember that the satellite that took the photos is unlikely to be directly overhead that plot so look at the closest buildings and see which way they are sloping and by how much so you can draw the polygon line in the right place (e.g. on the right side of the trees to be where the boundary is).
That said I would definitely recommend spending the money on a GPS survey by a reputable company - you may even be able to share the costs with the seller or your mate - the value in terms of peace of mind is priceless!!
I also absolutely agree with the other comments about regretting the sale of part to your mate - you might want to have your solicitor add something about if your mate (and any future owner) decides to sell it they must offer it to you or the future owner of your property, at a fair market rate of course, and most sales like that have some kind of covenant that say they can't use it for intrusive things.
Also think very carefully about your and your mate's access rights - that is something that if it becomes a problem can cause a lot of grief. When we bought a previous property we had no intention of applying for planning to extend but other external unforeseen factors meant we needed to apply for planning permission - so just think very carefully about what could become something you might do in the future - like if to get to your mate's property they have to come over your land make sure you have thought about what that might prevent you doing in the future - our neighbours are going through a nightmare to get secondary access to their plot because the original access they are using is causing friction with the landowner for no faualt of theirs because the landowner changed - so think what someone else might do if its not your mate and specifically if your mate does sell it on before you have the funds how that could affect you. Even a boundary dispute is something that should be declared on sale these days.
That said I would definitely recommend spending the money on a GPS survey by a reputable company - you may even be able to share the costs with the seller or your mate - the value in terms of peace of mind is priceless!!
I also absolutely agree with the other comments about regretting the sale of part to your mate - you might want to have your solicitor add something about if your mate (and any future owner) decides to sell it they must offer it to you or the future owner of your property, at a fair market rate of course, and most sales like that have some kind of covenant that say they can't use it for intrusive things.
Also think very carefully about your and your mate's access rights - that is something that if it becomes a problem can cause a lot of grief. When we bought a previous property we had no intention of applying for planning to extend but other external unforeseen factors meant we needed to apply for planning permission - so just think very carefully about what could become something you might do in the future - like if to get to your mate's property they have to come over your land make sure you have thought about what that might prevent you doing in the future - our neighbours are going through a nightmare to get secondary access to their plot because the original access they are using is causing friction with the landowner for no faualt of theirs because the landowner changed - so think what someone else might do if its not your mate and specifically if your mate does sell it on before you have the funds how that could affect you. Even a boundary dispute is something that should be declared on sale these days.