Paying it forward makes some mad

We got a wage garnishment against one of our bills, which was in another post, but anyhow, our in'laws are helping to pay our mortgage payment for a few months, and they are buying us food, and they are going to pay our taxes of $500 for the land and insurance til the garnishment stops (they are taking over $800 a month in garnishment). Well since they are helping us so much, they actually put us above a level we would be if we were paying all of our own stuff, and as they have an old well system for water, we had $150 left from the rest of the bills, so we thought we would buy them a water pump as gratitude for helping us in our time of need. My husband will install it. He installed their last one ten years ago, he is very soundly mechanically inclined.
When they called earlier to check up on us, because my husband is really sick and started medication last night, he mentioned the new pump being in the back of the truck but he wouldnt be able to install it tonight as if he needed more parts the parts store would be closed so he will do it tomorrow.
His mother went off the handle (if an angel ever could lol she is so sweet even when shes mad) she was upset that we went and bought it.
We tried to explain to her that they have helped us out so much by getting the loan for our house last year so we didnt have to rent anymore, and now helping us through another hard time so completely that they left us with the excess so we were not in need of anything so we would in return being helping them, as they live on a fixed income!!!!!!!!
She was upset. I dont understand, We are not going to take it back to the store as last year they bought us new tires for the truck that cost them over $400!! Everytime we need something they are there, and they are spending all thier money on us, they dont have any left!!!
They cant take a shower or wash their hands or do dishes unless they turn the water on for a few minutes and it spews (sp) water out of about 20 holes and saturates their wood shed where the water pump is located. And there is electrical stuff in there too!! They are keeping a HUGE pan on the stove full of water so they can wash their hands when they need to . I could go on and on, but just want to make it a point that it is no way to live, and we wanted to make them a little more comfortable as they have saved our butt way more then necessary!
What do ya'll think, should they be mad? We are going to install it no matter what, I just want to know if they are unjustifully angry, or are we in our right minds lol?