house built in 70's, sinking in the middle

First thing to do go underneath and see what's happening.
Could be rotting wood, sinking foundation, poorly built foundation, termites, fire damage?

Slab, crawlspace or basement? Some pictures would help. Thanks.

Look at foundation and load bearing supports. See if supports are sound, type of supports, termite damage, etc. Try to figure out what's causing it to sink in the middle.

You could hire a structural engineer to do the inspection, if that's an option for you. Price varies, call locally. Around here it runs anywhere from 300-500 dollars. Just a suggestion.

In addition to above questions and comments...you said she recently bought the house. Has this problem started since she bought it? Or was there evidence prior? How long ago did she buy it? or how long ago did it start?
how big is the lake? What feeds the lake? a stream/river? or is it spring fed?
How many other houses on the lake?

Most common cause, lack of proper footings under the piers.
Lake house makes me think built on the weekends by friends, no permits.
At least that how it is around here.