[David L.]

The direct speech is:
They’ve fixed it so it’s impossible to tamper with it unless you’re a ruddy watchmaker...

Second Q&A – she doesn’t know if she should use ‘is’ or ‘was’.
1. What did Matilda’s father say was impossible to do with the speedometer.
Matilda’s father said that it is/was impossible to ....

It's quite clear. He said it was impossible then and we have no information from the passage or common knowledge to say that the information has changed. Therefore, it is as true today as it was when it was said : Matilda’s father said it is impossible to tamper with...

compare:
"It is inconceivable that any rational Englishman would allow women to vote."
He said that it was inconceivable...

HOWEVER: Note that your friend's wording of the questions is determining the tense:

1. What kind of car did Matilda’s father say he would buy?
Matilda’s father said he would buy old cars that have about a hundred andfifty.....

2. What kind of car did Matild’s father say he buys?
Matilda’s father said he buys old cars....

Second Q&A – she doesn’t know if she should use ‘is’ or ‘was’.
1. What did Matilda’s father say was impossible to do with the speedometer.
Matilda’s father said that it was impossible to ....
(the answer to this question is, quite reasonably, "To tamper with it." The question is not changing direct into indirect speech, but one of elementary comprehension! )