My wife has the same problem. Since I've only been playing guitar for a little over 2 1/2 years it gets to be a problem for me to find the right key. I've learned, thru trial and error, that my wife is more comfortable singing in the key of "G" as she can get more projection without straning. What I do is look for the song under the Chordie format and either change the key thru the transpose function or, again by trial and error, I've learned to transpose the chords myself. I don't know the key of the song you mentioned but I do know the song. To transpose the chords I find its easer to use the "Nashville Numbering" system and assign all chords a number. If the song is in the key of "D" the chords would be:
1..........2..........3..........4..........5..........6..........7..........1
D..........E......F#/Gb......G..........A..........B.....C#/Db......D
To transpose the chords to the key of "G" the chords would be;
1.........2..........3.........4.........5..........6..........7..........1
G........A..........B.........C.........D.........E......F#/Gb.......G
In each of these progressions the 1st, 4th and 5th chords are Major chords.
The 2nd, 3rd and 6th chords are minor chords.
The 7th chord is a dimished chord.
This principle will work for all chord progressions so give it a try.
By using a capo you can change the key of a song while using chords you are familar with. An example of this would be;
If you know how to play in the key of "E" in a 1 - 4 - 5 progression the chords would be E, A and B. (easier to play a B7 chord)
By placing a capo on the 1st fret you can play the chords of E - A - B7 but the chord voicings would be F - Bb - C7.
By moving the capo to the 3rd fret when you play the chords E - A - B7 the chord voicing would now be G - C - D7.
Placing the capo on the 5th fret same chords but you are now playing in A.
This goes right on up. To use a capo or to learn to transpose chords it just takes a little time and effort. It is worth it!
There are better qualified people than me in this forum that can help you with this but I hope you can get something from all my rambling.
Nela