Mobilizing for a few months? Hmm ...
They were not at real war, because there were secret negotiations, Hitler's attempts to make peace with the rest of the West. Hitler was an Anglophile and admirer of Britain. He did not want war with Britain. He just wanted to be recognized as an equal with the rest ot the West.
Then how come Britain rejected Hitler's peace offers?
Can you tell me why Britain did not allow Hitler to occupy Poland but Stalin did allow Hitler to invade Poland?
If there were secret negotiations between Nazi Germany and Western powers to overthrow the Soviet Union then why Britain did not accept Hitler's peace offers between 1939-1941??
Yes, i agree, Hitler was an Anglophile, but does that change the fact that Britain still rejected his peace offers? No, it doesn't.
The victory over Japan was not so simple. If fights were close to the sea, for example, in Primorye, Japanese would have had every chance to win. The Japanese surrendered in large part precisely because Hitler made a pact with Stalin. Thus, the idea of a war against the Soviet Union on 2 fronts loses its meaning, and the Japanese could not afford a full-fledged war against the Soviet Union and China..
But we are talking about land forces. Soviet land forces were superior to any other in the world, including Germany's.
Secondly, the Hitler-Stalin pact was signed
after the Japanese were defeated at Khalkhin Gol. So therefore the Hitler-Stalin pact did not play any role in the Japanese defeat.
Soviet Red Army was an offensive army, superior to any other army in 1939-1941:
KV-1 was adopted in December 1939. T-34 was not yet at all.
Even if KV-1 was adopted in December 1939, no other country still had a comparable tank. Germany didn't have a comparable tank until late 1942.
Secondly, regarding the T-34, even if the Red Army didn't receive the T-34 until 1941,
does that change the fact that no other country had an equivalent of such a advanced offensive warfare weapon? In 1941 Germany did not have a single tank comparable to the T-34.
Even if the Red Army didn't receive T-34's until 1941, they still had thousands of the BT-7's and T-26 tanks which were superior to 80% of the tanks the Germans had.
In 1941, the most common Soviet Tank was the BT-7:
In 1941 the most common German tank was the Panzer 1:
Airplanes of the Soviet Unionare were inferior to German planes in the early stages of the war.
Rather, on the contrary they were superior to anything the Luftwaffe had:
The Soviet Su-2 attack plane was superior to the Stuka. The Luftwaffe had nothing comparable.
In 1939, The Soviets had the TB-3 heavy bomber, the Germans had no heavy bomber at all.
In 1939, The Soviets had the DB-3 bomber, the Germans had nothing comparable. In June, 1941 alone, the Soviet air force had 1,846 DB-3Fs bombers, more than the number of all types of bombers Hitler attacked the Soviet Union with.
In 1939, the Soviet air force had the Er-2 bomber with range of 4,000 km, the Luftwaffe had nothing comparable. In fact, until the end of the war the Germans had nothing comparable to this air plane.
And the Soviets also had other new model aircraft like the Pe-2's, Mig-3's, Yak-1's, LaGG-3's, Ar-2, Er-2, Pe-8, Yak-2, Yak-4, and Il-4.
These were all superior or equal to anything the German Luftwaffe had between 1939-1941 and in most cases even till the end of the war the Luftwaffe had nothing comparable to most of these aircraft, not even the rest of the world had anything comparable to these air crafts up till the end of WW2. So how can you say Soviet air force was inferior?
The British gave Hitler the whole of Europe almost without a fight, with the sole purpose - for Hitler to attack the USSR. Thus, the Anglo-Saxons eliminated 2 potential competitors, like in WW!. Hitler was a fool, that embodied the plans of the Anglo-Saxons in life.
But if the British wanted to eliminate two potential competitors then why did they send financial and military support to the Soviet Union?
Why did Britain and USA send billions of dollars worth of food stuffs and military equipment to save Stalin from defeat?
Im sorry for warning you late, i know you wasted alot of your precious time trying to convince him. in sorry. 
